Position control and indicating mechanism

ABSTRACT

A position control and indicating mechanism and method in which a control member, such as a disk, having a continuous generally spiral track, typically in the form of a groove, is used to return or advance a controlled device to a selected equilibrium position. A follower which rides in the groove, or otherwise follows the spiral track, is constrained to travel along a fixed, usually a generally axial path relative to the disk. The follower controls the operation of an output circuit, depending upon the position of the follower along its path, for example, its position relative to a null point on the control member. The output circuit can be used to cause rotation of the output member in a direction which moves the follower towards the null point, which corresponds to the equilibrium position of the device being controlled.

United States Patent Jackson, Jr.

POSITION CONTROL AND INDICATING MECHANISM Lewis B. Jackson, Jr., 31 Lake Shore Ct. Apt. 3, Brighton, Mass. 02135 Filed: Jan. 13, 1970 Appl. No.: 2,573

Related US. Application Data Continuation-impart of Ser. No. 791,392, Jan. 15, 1969, abandoned.

Inventor:

References Cited UNITED STATES PATENTS Feb. 15, 1972 Primary Examiner-Benjamin Dobeck Attorney-Dressler, Goldsmith, Clement & Gordon [5 7 ABSTRACT A position control and indicating mechanism and method in which a control member, such as a disk, having a continuous generally spiral track, typically in the form of a groove, is used to return or advance a controlled device to a selected equilibrium position. A follower which rides in the groove, or otherwise follows the spiral track, is constrained to travel along a fixed, usually a generally axial path relative to the disk. The follower controls the operation of an output circuit, depending upon the position of the follower along its path, for example, its position relative to a null point on the control member. The output circuit can be used to cause rotation of the output member in a direction which moves the follower position of the device being controlled.

12 Claims, 13 Drawing Figures GOA/TFO L I a5 c/lecon's 42 PAfliNrEnrzs 15 I972 SHEET 2 OF 4 n W 8 v I PATENTEOFEB 15 I972 SHEET 4 OF 4 fmve/xai POSITION CONTROL AND INDICATING MECHANISM This application is a continuation-in-part of my application Ser. No. 791,392, filed Jan. 15, 1969, now abandoned.

BACKGROUND As is well known, numerous devices are required to assume selected positions. Such positions can either be equilibrium positions, or may be one of a series of positions in which the device being controlled is to assume.

Typically, existing position control devices are either very limited in their operation and range, such as a cam formed as a part of a shaft, the position of which is being controlled, or, if a wider range of operation is desired, are relatively complex configurations requiring multiple stages, precision gearing and parts constructed to close tolerances.

It is of course desirable to have a device which can selectively and repetitively return a controlled device to an equilibrium position or which alternatively can select a plurality of successive positions which the control device is to assume. In either case, it would be desirable to have the capability to readily modify or change the desired position to which the controlled device is moved.

SUMMARY OF INVENTION In accordance with the present invention there is. provided a position control and indicating mechanism and method which can select positions with precision without the necessity of utilizing expensive precision parts and which also has a capability of modification and alteration for adaptation to varying uses.

The position control and indicating mechanism of the present invention is not limited to controlling rotational position or any specific form of positioning, but is capable of generating various control signals which can be used as desired in positioning a controlled device.

More specifically, the mechanism of the present invention includes one or more control members, typically disks, each having a continuous generally spiral track formed on the surface thereof. In the case of disks, this track extends from the periphery towards the axis of each disk. Typically, the spiral track may take the form of a groove etched into the surface of the control member. A null point or discontinuity associated with either or both the control member and a follower device defines the point used to select the desired position. The null point, or discontinuity may be formed in the control member, in the follower device or in both. In one embodiment, the null point takes the form of an offset portion formed in the track of the control member.

The follower is constrained to travel along a fixed path, e.g., in the disk embodiment, the follower usually travels along an axial path relative to the disk. The follower rides in the groove or otherwise follows the generally spiral track. Typically the follower is in contact with a suitable output circuit member, which may take the form of a movable contact arm or an electrical output member.

A contact arm suitable for use in conjunction with the mechanism of the present invention may take the form of a contact arm biased against the surface of the follower. The contact arm may have an offset portion the position of which is selected to coincide with the null point on the control member. The free end of the contact arm completes an electrical circuit to a suitable output device, for example, through one of two fixed contact points.

Thus, when the follower is in engagement with the spiral track on one side of the null point, the contact arm will close the circuit to the output device, such as a motor fordriving the control member, through one fixed contact point to drive the control member in a direction to cause the follower to approach the null point. When the follower is on the opposite side of the null point, it will cause the contact arm to engage the second fixed contact point thereby reversing the circuitry through the motor to drive the control member in an opposite direction thereby again causing the follower to move towards the null point. As the follower traverses the null point, it also engages the offset portion of the contact arm to open the circuit to the motor.

Thus, no matter which side of the null point the follower is initially positioned, energizing the control circuit causes operation of the control member until the follower reaches the null point, at which point the control circuit is deenergized. By coordinating the operation of the control member to movement of the device being controlled, the device can be returned to its desired equilibrium point.

The flexibility of such a system is readily apparent. For example, a plurality of tracking members can be utilized with a single control member. Depending upon which control circuit is energized, the controlled device may be advanced or moved to any selected position. In addition, a plurality of control members, each with single or multiple followers and control circuits can be interconnected with the null point on each control member selected to occupy any position relative to the null points on the other interconnected control members, thereby providing another embodiment by which a plurality of positions can be selected.

It should be quite apparent that by the present invention null points canbe readily selected simply by interchanging control members in which the null points in the spiral tracks are located at different positions in the track. Thus, simply by activating different control circuits, the position of a controlled device can simply and readily be selected and the device returned or advanced to that position.

In addition to the discreet control circuits described above, the contact arm can be replaced by other output members which can, for example, generate a continuous signal representative of the axial position of the follower and therefore representative of the position of the control member and of the controlled device. Such an output member can take numerous forms. It may be a continuous conductive member; it may be a member of varying resistance; it may combine discrect areas of different resistances so that the control signal generated as a result of the axial position of the tracking member along the disks may take a number ofdifferent forms.

Numerous other advantages and features of the present invention will become readily apparent from the following detailed description of the invention and of one embodiment thereof, from the claims and from the accompanying drawings in which each and every detail shown is fully and completely disclosed as a part of this specification, in which like numerals refer to like parts.

FIG. 1 is a diagrammatic plan view of one embodiment of the present invention;

FIG. 2 is a side view showing a plurality of disks ganged together for multiple operation;

FIG. 3 is a diagrammatic plan view showing multiple followers in combination with one disk;

FIGS. 4, 5 and 6 show alternative embodiments of the present invention;

FIGS. 7, 8 and 9 show a number of different type output members which can be utilized with the present invention;

FIGS. 10 and 11 show two embodiments of how the disk of the present invention may be selectively positioned and driven; and

FIGS. 12 and 13 show another embodiment of the present invention.

Referring now to the drawings, there is shown a number of embodiments of the position control and indicating mechanism of the present-invention. Referring to FIG. 1, there is shown a control member in the form of a disk 20 having in its surface a continuous generally spiral track 22, which typically takes the form of a spiral groove formed in the surface of the disk 20. The track 22 extends generally from the periphery 24 of the disk 20 inwardly to the axis 25 of the disk. The spiral track 22 includes a generally radial offset portion 26 which defines a null point in the disk 20. The disk is mounted on a suitable rotatable shaft 28 which may be driven to rotate the disk 20 as explained in more detail below.

A track follower 30, which in the case of a grooved disk may be quite similar to a phonograph needle, rides in the grooves of the disk or otherwise follows the track 22 and is constrained by a guide member 32 to travel along a fixed path relative to the disk 20. For purposes of clarity, this path will be described as a generally radial path, but it should be understood that the follower can be constrained to follow any desired path.

The embodiment of FIG. 1, includes a contact arm 34 having an offset portion 36 which coincides radially with the null point 26 defined by the track 22. The free end 38 of the contact arm 34, which may conveniently extend beyond the periphery 24 of the disk 20 is positioned between two fixed contacts 40, 42 each of which is connected to an output circuit 44 such as for example a motor.

The operation of the position control and indicating mechanism as disclosed in FIG. 1 is such as to cause rotation of the disk 20 in a direction to move the follower along its axial path towards the null point 26 in the disk 20, and therefore, towards the offset portion 36 in the contact arm 34. When the follower 30 is on one side of the null point 26, the contact arm 34 will complete a circuit from a source of energy, such as a battery 46 through the first contact 40 to the output circuit or motor 44. The motor 44 rotates the shaft 28 and the disk 20 in a direction to move the follower 30 towards the null point 26. If the follower 30 is on the opposite side of the null point 26, the contact arm 34 will complete the circuit to the output circuit 44 through a second contact 42 thereby reversing the motor 44 to rotate the disk 20 in an opposite direction, again causing the follower 30 to move towards the null point 26.

As the follower 30 engages the offset portion in the track 22 which defines the null point 26, it also engages the offset portion 36 of the contact arm 34 causing it to disconnect the battery 46 from the output circuit or motor 44. As a result the disk 20 stops rotating in its desired position.

By connecting a device to be controlled 48 (see FIG. 11) to the disk 20, so that the position of the controlled device 48 is a function of the rotational position of the disk 20, the controlled device can readily be returned to any selected position. Thus, assuming that the controlled device 48 has been moved from its desired equilibrium position, and that therefore the follower 30 is no longer at the null point 26 in the track 22, the controlled device 48 may be returned to its equilibrium position simply by closing the circuit through the contact arm 34. Since the arm will complete the circuit through one of the fixed contact 40, 42 to cause rotation of the controlled device 48, and the disk 20 through drive wheel 50 in the desired direction, the follower 30 will move towards the null point 26, as explained above, thereby returning the controlled device 48 to its equilibrium position.

Referring to FIGS. 2 and 3 a number of equilibrium positions can be selected by utilizing a plurality of disks 20, 20, 20 ganged to a common shaft 28, the null point 26 in each disk being different than the other disks. Selection of different null points is effected simply by activating a circuit through the contact arm associated with each respective disk. In this regard, the fixed contacts 40, 42 points can be used for all of the disks by running parallel to the shaft 48 and adjacent to the periphery 24 of the disks.

If minor variations in the position of the control device are desired, it is not necessary to utilize a plurality of interconnected disks 20. As seen in FIG. 3, a plurality of followers can be utilized in conjunction with a single disk 20, there being three followers 30, 30a, 30b, associated contact arms 34, 34a 34b, and contacts 40, 40a, and 40b, and 42, 42a, and 42b, being shown. It can be appreciated that by connecting the battery 46 to one of the contact arms 34, 34a, or 34b, the selected position can be varied within small increments.

Alternatively, if instead of a fixed position, it is desired to modify the output gradually, a contact arm of varying shape such as arm 52, (FIG. 4) can be utilized. In this embodiment, a follower 30 moves along its axial path as constrained by guide member 32 causing the free end of the contact arm 52 to traverse a suitable output circuit such as resistor 54. Thus, the mechanism of the present invention can be utilized to gradually modify the position of a controlled device or alternatively to indicate the position of such a device. A null point in this embodiment is shown as discontinuity 55 in track 22.

It should be understood that the control member for use in the mechanism of the present invention is not limited to a disk configuration. Thus, in FIG. 5, there is shown a belt 56 having a generally spiral track 58 formed therein which track includes, as does the track 22, an offset portion 60 defining a null point on the belt 56. The tracking member 30 which rides on support member 32 as well as the contact arm 34 function in the same manner as described above with respect to the disk 20.

If desired, the contact arm can be eliminated by connecting the battery 46 directly to a conductive follower 62, as seenin FIG. 6. In this embodiment the track 22 incorporates a pair of continuous conductors 64, 66 separated by a discontinuity in the form of a nonconductive portion 68 formed in the track at offset portion 26. Thus, the source would be connected to the output circuit 44 through the conductive follower 62, one of the conductive portions 64, 66 of track 22, through the feelers 71, 72 connected to the conductive portions 64, 66, respectively.

A conductive feeler can be used in conjunction with output members other than a contact arm 34 or the conductive track 64, 66. Referring to FIG. 7 there is shown a linear conductive output member 78 which, typically, is in the form of a resistor. The conductive output member 78 is disposed along the path of the follower 30, which in this embodiment would be conductive. Batteries 80, 82, of equal and opposite polarities are connected between opposite ends and the center of the conductive output member 78. The center of the output member is also connected to one terminal of the output circuit 44, the other terminal of which is connected to the follower 30 which is electrically connected to the output member 78 by a slide 84.

Thus, as the follower travels along its path, the signal input to the output circuit 44 varies in magnitude and polarity depending upon the position of the follower 30 along the output member. It can readily be recognized that the maximum signal occurs when the feeler 84 engages either end of the output member 78, while the signal decreases to zero as the feeler 84' reaches the center of the output member 78, which coincides with the follower 30 engaging the null point 26 on the disk 20 or other control member. The follower 30 traverses the null point 26 as the feeler 84 contacts the output member 78 in the area 86.

In the embodiment of FIG. 8, the circuit connections would be the same as for the embodiment of FIG. 7 with the exception that the output member 78 would be formed in segments of varying resistance. As shown, the outer most segment 88 is conductive while intermediate portions 90 of the output member 78' adjacent the center are of an electrically resistant material thereby decreasing the magnitude of the signal connected to the output device 44 at a rate different from the linear rate of the embodiment of FIG. 7. The output member 78', also includes at its center a zone of conductive material 92 connected to the output circuit 44.

In FIG. 9, a contact arm is replaced by a conductive output member 94, the opposite ends of which are connected to the output circuit 44 in a manner quite similar to the connection of the contacts 40, 42 in the previous embodiments. The center 96 of the output member 94, is nonconductive thereby disconnecting the battery 46 from the output circuit 44 as the follower 30 reaches the null point 26 along the track 22 of the control member which it is following.

FIGS. 10 and 11 disclose alternative embodiments for setting the desired position of a controlled device. In FIG. 10, the disk 20 is frictionally coupled to the shaft 28. With the follower in the null position, rotation of the disk is prevented by engagement of a stop member 98 in a corresponding slot 98 in the periphery of the disk 20. Thus, the disk is not rotated during subsequent movement of the controlled device as long as the stop member is in place. When the selected new equilibrium position is selected, the stop member 98 is disengaged from the slot 98 to allow operation of the device as set forth above.

Alternatively, as seen in FIG. 11, the drive wheel 50 which interconnects the controlled device 48 and the disk 20, can be moved out of contact with the devices while brake member 99 is moved into contact with the periphery of the disk 20 to hold it in position with the follower 30 at the null point 26. The controlled device may then be reset into its desired position, and the drive wheel 50 returned to operatively interconnect the controlled device 48 and the disk 20 for normal operation.

ln yet another embodiment as shown in FIGS. 12 and 13 the control member takes the form ofa disk 100 having a continuous generally spiral track 102 in the form of a groove formed in one surface 104 thereof, the groove 102 including a generally radial offset portion 106 defining a null point. The opposite surface 108 of the disk 100 includes a raised area 1 disposed to one side of the null point 106, e.g., disposed radially inwardly thereof and a lower level area 112 disposed outwardly thereof, the areas 110, 112 being interconnected by a discontinuity or ramp portion 114. I

A tracking member 116 disposed on one side of the disk 100 rides in the spiral groove 102, the tracking member 116 being formed integrally with a switch support member 118 on which is mounted a movable contact 120 riding on the surface 108, and two fixed contacts 122, 124. The integrally formed tracking member 116 and switch support member 118 are supported rotatably on a shaft 126 to allow generally axial movement thereof as the disk 100 is rotated.

The movable contact arm 120 engages the first fixed contact 122 when riding on raised area 110, on one side of the ramp 114, and engages the second contact 124 when riding on the lower level area 112 on the other side of the ramp 114. As the disk 100 rotates, the tracking member 116 and the switch support member 118, move toward the null point 106. When the null point 106, is reached, the switch contact 120 traverses the ramp 114 thereby opening the electric circuit. Continued rotation of the disk causes the contact 120 to engage the second contact 124 thereby reversing operation of the disk 100, as described above.

By disposition of the switching support member 118 on the opposite side of the disk 100 from the tracking member 116 allows for a track shallower grooves so that the grooves may be more closely packed together thus effecting a finer position control. In order to insure that the contact 120 engages the ramp 114 only at the point where the tracking member 116 is at the null point 106, the width of the groove spacing can be increased at that point of discontinuity. It is clear that although the embodiment of FIGS. 12 and 13 has been described with a discontinuity incorporated into the spiral groove, the use of separate tracking and contact systems permits the elimination of such discontinuity if desired.

Precision of the disk controller of the present invention as compared with that offered by a cam of equal switching radius should be noted. If it is assumed that the switches on both devices perform in response to the same angular rotation, and understanding that a cams range is one-half turn, the range of the disk as N turns, where N is the number of spiral grooves on either side of the null point. Thus, if a cam can locate a particular control device to within it inch of the desired position, a disk of the present invention could locate that device to within x/ZN, a substantial increase in accuracy.

From the foregoing, it will be observed that numerous variations and modifications may be effected without departing from the true spirit and scope of the novel concept of the invention. It is, of course, intended to cover by the appended claims all such modifications as fall within the scope of the claims.

What is claimed is:

1. A position control and indicating mechanism comprising in combination a control member, said control member defining a generally spiral continuous track, a discontinuity associated with said control member, follower means in engagement with said spiral track, means slidably supporting said follower means for movement along a generally fixed linear path that intersects said spiral track, means for driving said control member to move said follower means along said fixed path to traverse said discontinuity, and output means responsive to the position of said follower means along said path for generating output signals representative of the relative position of said follower means and said control member with respect to each other.

2. The mechanism of claim 1 in which said track includes an offset portion intermediate its ends to define said discontinuity, and in which said output means generates a first output signal in response to engagement of said follower means with said track on one side of said discontinuity, generates a second output signal in response to engagement of said follower means with said track on the other side of said discontinuity, and generates a third output signal in response to engagement of said follower means with said track at said discontinuity.

3. The mechanism of claim 2 in which said output means includes said drive means, means for connecting said drive means for driving said control member and said follower means relative to each other in a first direction in response to said first output signal, for driving said control member and said follower means relative to each other in a second direction in response to said second output signal, whereby said follower means travels towards said discontinuity, and for stopping movement of said control member and said follower means relative to each other in response to said third output signal when said follower means engages said discontinuity.

4. The mechanism of claim 1 in which said control member is a disk, and said generally spiral track extends from the periphery of said disk to the axis thereof.

5. A position control and indicating mechanism comprising in combination a control member, a continuous track in the surface of said control member, said track terminating in ends located at spaced-apart locations on said control member and defining a null point in the form of a discontinuity in said track intermediate its ends, said track having a plurality of revolutions extending between said discontinuity and each of its ends, follower means for following said track, means for slidably constraining said follower means to travel along a generally fixed linear path intersecting said track, means for driving said control member to move said follower means along said fixed path to traverse said discontinuity, and means responsive to the position of said follower means along said path for generating output signals representative of the position of said follower means with respect to said null point.

6. The mechanism of claim 5 in which said means for generating output signals includes means for generating a first control signal representative of the positioning of said follower means on one side of said null point, a second control signal representative of the positioning of said follower means on the other side of said null point, and a third control signal representative of the positioning of said follower means at said null point.

7. The mechanism of claim 5 in which said signal generating means includes a source of electrical energy, first, second and third contacts, and output means, means for connecting said source to said first contact, means for connecting each of said second and third contacts to said output means, means for connecting said first contact to said output means through said second contact when said follower means is on one side of said null point, for connecting said first contact to said output means through said third contact when said follower means is on the other side of said nuli point, and for disconnecting said first contact from said output means when said follower means is at said null point.

8. The mechanism of claim 7 in which said first contact is a contact arm which extends generally along said path and includes two segments laterally offset from each other and interconnected by an arm lateral offset portion aligned with said null point, and in which said means for connecting and disconnecting said contact arm to said output means includes means for biasing said contact arm against said follower means, whereby engagement of said follower means with said arm on one side of said offset portion causes said contact arm to engage said first contact, whereby engagement of said follower means with said contact arm on the other side of said offset portion causes said contact arm to engage said second contact, and whereby engagement of said follower means with the offset portion of said contact arm disengages said contact arm from both said first and second contacts.

9. The mechanism of claim 7 wherein said first contact is incorporated as a part of said follower means, said spiral track includes a first continuous conductor on one side of said null point defining said second contact, and a second continuous conductor on the other side of said null point defining said third contact, said continuous conductors being separated by a nonconductive portion defining said discontinuity.

10. The mechanism of claim 5 in which said null point is an offset portion in said track.

11. The mechanism of claim 7 wherein said first contact is supported by a contact arm disposed on the opposite side of said control member and in alignment with said follower means, wherein said means for connecting said first contact to said output means through said second contact includes a first, raised area on said opposite side disposed to one side of said null point, wherein said means for connecting said first contact to said output means through said third contact includes a second lower level area, and wherein said means for disconnecting said first contact from said output means includes a ramp portion interconnecting said raised and said lower level areas.

12. The mechanism of claim 11 wherein the spacing of said track is increased in the area of said null point. 

1. A position control and indicating mechanism comprising in combination a control member, said control member defining a generally spiral continuous track, a discontinuity associated with said control member, follower means in engagement with said spiral track, means slidably supporting said follower means for movement along a generally fixed linear path that intersects said spiral track, means for driving said control member to move said follower means along said fixed path to traverse said discontinuity, and output means responsive to the position of said follower means along said path for generating output signals representative of the relative position of said follower means and said control member with respect to each other.
 2. The mechanism of claim 1 in which said track includes an offset portion intermediate its ends to define said discontinuity, and in which said output means generates a first output signal in response to engagement of said follower means with said track on one side of said discontinuity, generates a second output signal in response to engagement of said follower means with said track on the other side of said discontinuity, and generates a third output signal in response to engagement of said follower means with said track at said discontinuity.
 3. The mechanism of claim 2 in which said output means includes said drive means, means for connecting said drive means for driving said control member and said foLlower means relative to each other in a first direction in response to said first output signal, for driving said control member and said follower means relative to each other in a second direction in response to said second output signal, whereby said follower means travels towards said discontinuity, and for stopping movement of said control member and said follower means relative to each other in response to said third output signal when said follower means engages said discontinuity.
 4. The mechanism of claim 1 in which said control member is a disk, and said generally spiral track extends from the periphery of said disk to the axis thereof.
 5. A position control and indicating mechanism comprising in combination a control member, a continuous track in the surface of said control member, said track terminating in ends located at spaced-apart locations on said control member and defining a null point in the form of a discontinuity in said track intermediate its ends, said track having a plurality of revolutions extending between said discontinuity and each of its ends, follower means for following said track, means for slidably constraining said follower means to travel along a generally fixed linear path intersecting said track, means for driving said control member to move said follower means along said fixed path to traverse said discontinuity, and means responsive to the position of said follower means along said path for generating output signals representative of the position of said follower means with respect to said null point.
 6. The mechanism of claim 5 in which said means for generating output signals includes means for generating a first control signal representative of the positioning of said follower means on one side of said null point, a second control signal representative of the positioning of said follower means on the other side of said null point, and a third control signal representative of the positioning of said follower means at said null point.
 7. The mechanism of claim 5 in which said signal generating means includes a source of electrical energy, first, second and third contacts, and output means, means for connecting said source to said first contact, means for connecting each of said second and third contacts to said output means, means for connecting said first contact to said output means through said second contact when said follower means is on one side of said null point, for connecting said first contact to said output means through said third contact when said follower means is on the other side of said null point, and for disconnecting said first contact from said output means when said follower means is at said null point.
 8. The mechanism of claim 7 in which said first contact is a contact arm which extends generally along said path and includes two segments laterally offset from each other and interconnected by an arm lateral offset portion aligned with said null point, and in which said means for connecting and disconnecting said contact arm to said output means includes means for biasing said contact arm against said follower means, whereby engagement of said follower means with said arm on one side of said offset portion causes said contact arm to engage said first contact, whereby engagement of said follower means with said contact arm on the other side of said offset portion causes said contact arm to engage said second contact, and whereby engagement of said follower means with the offset portion of said contact arm disengages said contact arm from both said first and second contacts.
 9. The mechanism of claim 7 wherein said first contact is incorporated as a part of said follower means, said spiral track includes a first continuous conductor on one side of said null point defining said second contact, and a second continuous conductor on the other side of said null point defining said third contact, said continuous conductors being separated by a nonconductive portion defining said discontinuity.
 10. THe mechanism of claim 5 in which said null point is an offset portion in said track.
 11. The mechanism of claim 7 wherein said first contact is supported by a contact arm disposed on the opposite side of said control member and in alignment with said follower means, wherein said means for connecting said first contact to said output means through said second contact includes a first, raised area on said opposite side disposed to one side of said null point, wherein said means for connecting said first contact to said output means through said third contact includes a second lower level area, and wherein said means for disconnecting said first contact from said output means includes a ramp portion interconnecting said raised and said lower level areas.
 12. The mechanism of claim 11 wherein the spacing of said track is increased in the area of said null point. 